Corundum
4 of the 6 encyclopedias on this shelf carry an entry for Corundum. All 4 are reproduced below, so you can see where they agree and where they differ.
The Encyclopedia of Founding (1892)
A mineral almost equal to the sapphire in hardness, and for that reason it is employed, like emery, for polishing metals and hard stones. It is pulverized, mixed with glue, and other gum compositions, and moulded into all forms of wheels for grinding and polishing every variety of articles produced in the metal industry. The colors of corundum are various, including green, blue, red, inclining to gray, of a shining lustre, translucent or opaque; specific gravity, 4; fusible only by the compound blowpipe. The composition of corundum is: crystalline alumina 85.5, silex 7, oxide of iron 14. It is found in the East Indies, China, the United States, and other places. The Chinese varieties are called adamantine-spar. See EMERY; PRECIOUS STONES.
Aiton's Encyclopedia (1910)
a well known mineral. It is a compound of aluminum and oxygen. It is a compound of aluminum and oxygen. It is hard, ranking next to the diamond. There are three recognized types of corundum. The first is the gem called the sapphire. Five varieties are known. A second type is corundum proper. It differs from the gems only in lack of bright color. It is used as a polishing material. The third form is called emery, which is merely an impure corundum. There is an important corundum deposit north of Kingston, Ontario. Supplies are obtained also from New Jersey and the Carolinas. See Sapphire ; Emery ; Grindstone
Collier's New Encyclopedia (1921)
a rhombohedral transparent or translucent mineral, very tough when compact. Its hardness is 9, its sp. gr. 3.9-4.16. Its luster is generally erally vitreous; vitreous; its colors blue, red, yellow, brown, gray, or nearly white; its streak in all cases colorless. It consists of pure alumina. Chemically viewed, it is aluminum-oxide, Alâ‚‚Os. There are three varieties of it-sapphire, corundum proper, and emery. It includes the species of the genus which are dark in color and only translucent; but its hues may be light blue, gray, brown, or black. The United States imports corundum to the value of $100,000 every year, principally from Canada.
American Watchmaker and Jeweler (1892)
The earth alumina, as found native in a crystalline state, including sapphire, which is the fine blue variety; the oriental ruby or red sapphire; the oriental amethyst, or purple sapphire. It is the hardest known substance next to the diamond. The non-transparent variety, dark-colored and granular is known as emery. See Carborundum.
What Has Changed in the Last Century
The chemistry settled quickly: the 1892 Encyclopedia of Founding gave corundum as "crystalline alumina 85.5, silex 7, oxide of iron 14" (a total over 100 per cent), while Collier's in 1921 correctly called it pure aluminium oxide, Al2O3, which is what it is. The bigger change is that corundum no longer has to be dug up. Verneuil's flame-fusion process, already working in Paris by 1902 but unmentioned in any of these entries, made cheap synthetic sapphire and ruby routine, and today grown corundum turns up as watch glasses, scratch-resistant phone camera covers, LED substrates and bearing jewels; Theodore Maiman's first working laser, in 1960, used a synthetic ruby rod. Natural corundum and emery as abrasives were largely displaced by manufactured silicon carbide and fused alumina, so the Ontario and Carolina deposits the 1910 and 1921 accounts point to are no longer commercially important.
Written for this edition, 2026.